setup.py: honor CMAKE_GENERATOR_PLATFORM for the Windows -A flag (#592)

The Windows build hardcoded the Visual Studio target architecture to x64
for any 64-bit interpreter (`if sys.maxsize > 2**32: -A x64`). On
win-arm64 the interpreter is 64-bit but the architecture is ARM, so this
mis-targeted the build and find_package(Python) then failed. Honor an
explicit CMAKE_GENERATOR_PLATFORM (e.g. "ARM64") from the environment,
falling back to the previous behavior otherwise.

Same as openPMD/openPMD-api#1910

Author: ax3l

setup.py

@@ -156,7 +156,10 @@ def build_extension(self, ext):
                     cfg.upper(), os.path.join(extdir, "amrex")
                 ),
             ]
-            if sys.maxsize > 2**32:
+            generator_platform = os.environ.get("CMAKE_GENERATOR_PLATFORM")
+            if generator_platform:
+                cmake_args += ["-A", generator_platform]
+            elif sys.maxsize > 2**32:
                 cmake_args += ["-A", "x64"]
         else:
             cmake_args += ["-DCMAKE_BUILD_TYPE=" + cfg]